1. Field of the Invention
The present invention generally relates to packages for microwave circuits, and, more specifically, to a surface mount hermetically sealed package useful of the range of 0.1-50 GHz.
2. Description of the Prior Art
In the growing market of personal and satellite communications there is a need for a low cost, wideband, surface mount package (SMT) that can be used in a host of different environments. To provide maximum versatility or flexibility, such packages needs to be user friendly, and reliable and secure and protect microwave monolithic integrated circuits (MMIC) and other devices. Because such circuits can be used in satellites, be exposed to varied climatic conditions as well as possibly corrosive and polluting contaminants in the environment, such packages should also preferably be hermetically sealed to protect the integrated circuits housed within the package. Also, since applications have been identified for use of the microwave range of up to 50 GHz and beyond a package of this type must be suitable for use over broadband ranging from DC upwards of 50 GHz. In the past, IC devices have been packaged using complex, high cost packages, designed for low volume assembly and frequently without the requisite high reliability.
While some existing packages are suitable for surface mounting, these typically employ expensive materials and use glass-to-metal seals, multi-layer ceramic structures, machined metallic cases of substrates, expensive alloys and relatively thick gold plating. Some of these packages use the following technologies: stripline, microstrip, coaxial, and coplanar waveguide transmission line structures. However, some of these conventional packages introduce discontinuities in the electromagnetic field distributions and RF current configurations. These discontinuities need to be compensated for in prior art packages to eliminate signal reflections and losses that would otherwise be damaging to the performance of the package.
One of the difficulties with surface mount (SMT) packages at the higher microwave frequency ranges is that the packages inevitably have a bottom wall, which makes contact with the printed circuit board (PCB), and such bottom wall has a predetermined thickness. Therefore, there must be a “stepped” transition to bring the signal to the circuitry within the case or package at a higher physical plane above the bottom surface of the case or package that makes physical contact with the conductive pads or lands on the PCB. This presents an impedance matching issue which, as the frequency increases, can become more significant and problematic.
In an article entitled “50 GHz Broadband SMT Package for Microwave Applications”, Yoshida et al., “2001 Electronic Components and Technology Conference” a discussion is presented of several SMT packages. However, the authors recognize that at frequencies over 30 GHz the return loss in an SMT package is frequently degraded, and that such a package that achieves transmissions of up to 50 GHz and above with low loss has still not been reached or achieved. Such a “step up” or displacement in physical planes required by SMT packages is frequently achieved by the use of holes or VIA through a PCB or package, that are typically drilled with tiny drill bits and filled with pins or rivets. However, when very small VIA's are required, drilling with mechanical bits is costly because of high rates of wear and breakage. In this case, VIAs may be evaporated by lasers. Laser-drilled VIAs typically have an inferior surface finish inside the hole and these holes are called micro-VIAs. The quality of the VIAs, in some instances, limits the quality of the hermetic seal that can be achieved for the package.
U.S. Pat. No. 5,853,295 to Rosenberger discloses a coaxial line cut away at an angle for attachment to a substrate and a planar to coaxial connection, where the coaxial line is cut away at about a 60 degree angle so that the flat section meets a microstrip line at about 30 degrees.
Other references that suggest a step up include U.S. Pat. No. 6,987,429 and U.S. Published Patent Application No. 2002/0186105, both to Shih et al., which are for a waveguide to microstrip connection. This patent and patent application disclose an interconnection between a waveguide and a microstrip, comprised of a “J”-shaped plate. The plate has a bifurcated end to the base of the J, where it is attached to a probe extending from the waveguide. The J is more like an L with a corner cut off. However, the J does not vary in width along is height transition, and the connection is not from coaxial to circuitry. These are clearly “J” strips, but they are not shown as coaxial to circuitry connection. The input has a bifurcated connection to a rod extending from the waveguide. Possibly this could be the inner wire of a coaxial line.
U.S. Pat. No. 6,018,283 to Kyhl et al. discloses a Z-axis interconnector of overlapping striplines with tuning stubs, which discloses a two-axis interconnect, i.e., Z axis, used in MMIC, comprised of multi-layer stripline segments, overlapping, with tuning stubs. The frequency range is DC to 100 Ghz. U.S. Pat. No. 4,280,112 (Eisenhart) discloses a coaxial to stripline connection, with the center lead in the coax tilted downward and the bottom third of the coaxial section cut away. U.S. Pat. No. 5,936,492 (Shingyoji et al.) discloses a MMIC interconnection in the form of an upward sloping stripline over a dielectric (for impedance matching). The frequency range is DC to 100 GHz. U.S. Patent/Publication Nos. 6,215,377, 6,426,686 and 2002/0075105 (all by Douriet et al.) all disclose microstrip interconnections with step-up sections. The '377 patent uses VIA's for the step-up. The '686 patent and the '105 patent application use strips for the step-up. U.S. Patent/Publication Nos. 6,661,318 and 2003/0206084 (by Tamaki et al.) disclose coaxial lines used as interconnections for different height circuitry. The center lead id soldered to a lower plane surface, the shield is soldered to higher plane surface, and the line is fed to a still higher level. U.S. Pat. No. 6,737,931 (Amparan et al.) discloses a sloped and tapered strip transmission line for interconnections at different heights. U.S. Pat. No. 6,980,068 (Miyazawa et al.) discloses a coaxial to stripline transition where the center conductor is attached to the central plane of a three layer sandwich and the outer layers are attached to the outer coaxial shield. U.S. Pat. No. 6,992,544 (Barnes et al.) discloses a coaxial SMT connection, with the coaxial line at right angles to the SMT plane. U.S. Publication No. 2004/0051173 (Koh et al.) discloses a coaxial cable to a shelf mounted microstrip. U.S. Publication No. 2005/0285234 (Kanno) discloses an interconnection comprised of a series of striplines and VIA's, impedance 50 ohms, frequency range up to 45 GHz.